FIG. 25 shows an example of the system arrangement of a conventional digital copying machine.
Referring to FIG. 25, reference numeral 4001 denotes an image scanner as an apparatus for scanning a document image; and 4002, a printer for outputting a document image scanned by the image scanner 4001. The printer 4002 includes a controller for rasterizing print data (e.g., PDL data) sent from a computer (to be described later), and processes a print request from the computer. The controller can execute a process for outputting the document image scanned by the image scanner to the computer apparatus, thereby providing a function of a network scanner.
Generally, the network scanner processes images in a compression format such as JPEG or the like.
Reference numeral 4003 denotes a scanner connection cable. The scanner connection cable 4003 connects the image scanner 4001 and printer 4002, and its specification differs depending on manufacturers.
Reference numeral 4004 denotes a control panel which is controlled by the controller of the printer 4002, displays information such as designated, trimming, and color conversion of a scanned image, status monitor of expendables, and the like in addition to the designated copying or network scan operation, and allows the user to interactively make various setups.
Reference numeral 4005 denotes a control panel cable for connecting the control panel 4004 and the controller of the printer 4002; 4006a, 4006b, and 4006c, computer terminals; 4007, a printer server; and 4008, a network line.
Print data output from each of the computer terminals 4006a to 4006c is sent to the printer server 4007 via the network line 4008, and is then sent from the printer server 4007 to the printer 4006 via the network line 4008, thus executing a print process.
The digital copying machine system that adopts the aforementioned separate arrangement has the following features.
1. System Expandability
Since the image scanner 4001 and printer 4002 are separately arranged, the system can be expanded from a printer to a digital copying machine by adding the image scanner 4001 after the printer 4002 is purchased.
2. Connection Among Different Models
By standardizing interfaces of the image scanner 4001 and printer 4002, different models can be connected.
For example, full-color and monochrome image scanners can be selectively connected to the printer 4002.
Also, various models of full-color image scanners such as a full-color image scanner that can achieve high image quality, a full-color image scanner of cost-down version, and the like can be easily connected, thus allowing easy version up and exchange of the system.
3. Easy Development
Since the image scanner 4001 and printer 4002 can be independently developed, the development period can be shortened, and new functions can be easily added.
However, in the prior art, since the image scanner and printer are separately arranged, if different models are connected, the following problems may be posed:
1. tone reproduction mismatch
2. color reproduction mismatch
These problems are posed since individual image scanner models have different linearity characteristics.
Different linearity characteristics in units of models depend on the characteristics of devices such as a CCD image sensor, sample & hold circuit, gain adjustment circuit, A/D converter, and the like used in an image scan process.
Furthermore, the following problems may also be posed.
(1) image quality mismatch upon connecting different models
(2) image quality mismatch among individual units of identical models
(3) image quality mismatch after an optical unit is exchanged
“Image quality” indicates the quality of text and edges, and is determined especially by imaging performance of a light beam on a CCD line sensor. The causes and principle of generation of the problems will be explained below with reference to FIGS. 26 and 27.
FIG. 26 shows the path of a light beam from a document to a CCD imaging surface. Reference numeral 3001 denotes a platen glass on which a document (not shown) is placed. Reference numerals 3002, 3003, and 3004 denote first, second, and third mirrors. The first, second, and third mirrors 3002, 3003, and 3004 form an optical system for guiding light reflected by the document surface to a CCD line sensor 3006, and the first mirror 3002 and a unit of the second and third mirrors 3003 and 3004 are driven at a speed ratio of 2:1 in a scan direction while maintaining an optical path length upon scanning the document.
Reference numeral 3005 denotes an optical lens for imaging light reflected by the document on the CCD line sensor 3006, and is formed by building a plurality of lenses in a lens barrel. Reference numeral 3006 denotes a CCD line sensor which is a 3-line color sensor having three arrays, i.e., R, G, and B, photodiode arrays. The three photodiode arrays have a spacing corresponding to a width for eight lines on the document surface, and are arranged in the order of R, G, and B, as shown in FIG. 26.
Solid lines A, B, and C indicate ideal paths of light beams from the platen glass 3001 to the CCD line sensor 3006, i.e., paths of light beams that appropriately form images on the R, G, and B photodiode arrays. In case of solid lines A, B, and C, the R-G physical distance (8 lines) and G-B physical distance (8 lines) of the CCD line sensor 3006 match the distance for eight lines on the platen glass 3001.
On the other hand, broken lines α and β indicate an example of paths of light beams from the optical lens 3005 to the CCD line sensor 3006 when the optical lens 3005 suffers eccentricity.
Broken line α indicates the path of a light beam that forms an image on the R photodiode of the CCD line sensor 3006, and an image is formed on the inner side compared to the ideal state indicated by solid line A. This imaging position corresponds to the (7.891)-th line position with reference to the G photodiode, and this means that reflected light at the 8th line position on the surface of the platen glass 3001 forms an image at the (7.891)-th line position on the CCD line sensor 3006. Therefore, a light beam that forms an image at the 8th line position on the CCD line sensor 3006 is reflected light coming from the (8.1105)-th line (=8×8/7.891) on the platen glass 3001.
Likewise, broken line β indicates the path of a light beam that forms an image on the B photodiode, and this light beam forms an image at the (8.047)-th line position with reference to the G photodiode. Therefore, the light beam that forms an image on the B photodiode is reflected light coming from the (7.9533)-th line (=8×8/8.047) on the platen glass surface.
Such positional deviations of light beams that form images on the CCD line sensor 3006 are mainly caused by:
(1) tolerance and deformation upon assembling the lens barrel
(2) imaging adjustment error
(3) mirror precision
Therefore, the deviations of the imaging positions differ among not only models but also individual units. Even in a given individual unit, the deviation changes after an optical unit such as mirrors, lens unit, or the like is exchanged.
FIGS. 27A and 27B show influences of color misregistration (deviations of R, G, and B imaging positions of three colors). In general, image signals scanned by a 3-line color CCD line sensor undergo a line spacing correction process for correcting phase differences among R, G, and B.
In the example shown in FIGS. 27A and 27B, a given line on a document is scanned at time intervals for eight lines in the order of R, G, and B. A line spacing correction circuit delays a G signal for eight lines and an R signal for 16 lines with reference to a B signal which is scanned last, thus adjusting the phases of the three color image signals.
FIG. 27A illustrates an example of an image obtained when three color images are ideally formed. The scanning phase differences in the scan direction are perfectly corrected by the line spacing correction circuit, and the corrected R, G, and B image signals have no phase difference, thus reproducing a clear edge portion of letter “A”.
FIG. 27B illustrates an example of an image obtained when color misregistration has occurred. As has been described above with reference to FIG. 26, since the number of lines between R and G on the document is 8.1105, and that between G and B on the document is 7.9533, the distances between neighboring colors are:
R-B: 16.0638 lines
G-B: 7.9533 lines
with reference to a B signal.
Therefore, the phase differences after the process of the line spacing correction circuit are:
R-B: 16.0638−16=0.0638 lines
G-B: 7.9533−8=−0.0467 lines
These phase differences blunt the edge portion of letter “A” and generate false colors, as may be apparent from FIG. 27B.
As described above, color misregistration seriously influences especially text reproduction.
In general, digital copying machines and flatbed scanners use various types of light sources. Such light sources include, e.g., a halogen lamp, hot or cold cathode fluorescent lamp using mercury vapor, and the like. The halogen lamp is most prevalently used in digital copying machines, and has merits, i.e., allows adjustments of the light amount and light distribution and has stable light amount, color tincture, and the like. However, the halogen lamp requires large electric power since around 80% of consumed electric power are converted into heat, and is vulnerable to vibrations since light is emitted using a filament.
A flatbed scanner mainly uses a fluorescent lamp since it can assure low power consumption and long service life. In order to use a fluorescent lamp as an alternative light source of a halogen lamp in a digital copying machine with high productivity, an increase in efficiency of the fluorescent lamp has been extensively studied. Furthermore, the use of a fluorescent lamp as an alternative light source of a halogen lamp in a color digital copying machine that must meet a strict image quality requirement is beginning to be noted as a viable option.
There are some types of fluorescent lamps in terms of their structures, and typical ones will be explained below.
A hot cathode fluorescent lamp will be explained first. The hot cathode fluorescent lamp has filaments that emit thermions at the two ends of a fluorescent tube which contains mercury vapor, excites mercury by the emitted thermions, and converts the excited mercury into visible light by a phosphor applied on the inner wall of the tube. In this hot cathode fluorescent lamp, the amount of thermions to be emitted is controlled by a current supplied to the filaments, thus adjusting the light amount.
In a cold cathode fluorescent lamp, a high voltage is applied across electrodes at the two ends of a fluorescent tube to achieve gas separation. In general, a cold cathode fluorescent lamp uses mercury vapor, and is called by such name due to a smaller heat amount than the hot cathode type. Since the cold cathode fluorescent lamp is free from any wear of electrodes, its service life is much longer than the hot cathode type.
An external electrode type rare gas fluorescent lamp is represented by a xenon lamp. In this xenon lamp, xenon gas is sealed inside a tube, xenon atoms are excited by applying a high voltage across counter electrodes arranged outside the fluorescent tube, and the excited electrons are converted into visible light by a phosphor. This xenon lamp has a long service life since it has no expendable parts, but must be applied with a high voltage since it uses xenon gas which is harder to separate than mercury and requires an insulation treatment of the external electrodes. In general, since it is difficult to control a high voltage to be applied, the light amount cannot be adjusted over a broad range.
In this manner, the aforementioned lamps emit light according to the principle in which atoms sealed in the tube are excited and the excited atoms are converted into visible light by a phosphor. Hence, the optical characteristics of these lamps largely depend on the characteristics of the phosphor.
The emission spectral characteristics of a white xenon lamp will be described below with reference to FIGS. 28 and 29. FIG. 28 shows the emission spectral characteristics of a white xenon lamp, and FIG. 29 shows the relationship between the continuous ON time of the white xenon lamp and the output level of a CCD that receives light from the white xenon lamp.
The graph shown in FIG. 28 depicts waveforms obtained by normalizing the initial emission spectral intensity characteristics and emission spectral intensity characteristics for an accumulated ON time of 1,500 hours to overlap each other. In general, a fluorescent tube obtains white by combining a plurality of phosphors, and its emission spectral intensity characteristics have a plurality of peaks, as shown in FIG. 28. These white lamps are generally called 3-wavelength type, and have different characteristics depending on their manufacturers. Most of currently available white fluorescent tubes are of 3-wavelength type. As can be seen from FIG. 28, the characteristics considerably deteriorate in the neighborhood of 400 to 500 nm when the accumulated ON time is 1,500 hours compared to the initial characteristics. Such deterioration is caused by degradation of a phosphor.
Deterioration due to continuous ON operation will be explained below with reference to FIG. 29. In FIG. 29, the abscissa plots the continuous ON time, and the ordinate plots the CCD output. As can be seen from FIG. 29, all the R, G, and B CCD output levels deteriorate with increasing continuous ON time. Also, RGB balance changes. When an ON time of 500 hours has elapsed, RGB balance and CCD output levels stabilize.
However, in an image scanning apparatus such as a scanner that uses a white fluorescent lamp, since the RGB output balance of the CCD changes in correspondence with the accumulated ON time of the white fluorescent lamp, color reproducibility changes over time. As a result, deterioration of image quality such as image quality mismatch between identical models and deterioration of color reproduction occur.